1. Field of the Invention
The present invention relates to a gas sensor and a nitrogen oxide sensor for measuring oxides such as O2, NO, NO2, SO2, CO2, and H2O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as CO and CnHm.
2. Description of the Related Art
Those hitherto known as the method for measuring NOx in a measurement gas such as combustion gas include a technique in which the NOx-reducing ability of Rh is utilized to use a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia so that an electromotive force generated between the both electrodes is measured.
However, the sensor as described above suffers the following problems. That is, the electromotive force is greatly changed depending on the change in concentration of oxygen contained in the combustion gas as the measurement gas. Moreover, the change in electromotive force is small with respect to the change in concentration of NOx. For this reason, the conventional sensor tends to suffer influence of noise.
Further, in order to bring out the NOx-reducing ability, it is indispensible to use a reducing gas such as CO. For this reason, the amount of produced CO is generally smaller than the amount of produced NOx under a lean fuel combustion condition in which a large amount of NOx is produced. Therefore, the conventional sensor has a drawback in that it is impossible to perform the measurement for a combustion gas produced under such a combustion condition.
In order to solve the problem as described above, for example, Japanese Laid-Open Patent Publication No. 8-271476 discloses a NOx sensor comprising pumping electrodes having different NOx-decomposing abilities arranged in a first internal space which communicates with a measurement gas-existing space and in a second internal space which communicates with the first internal space, and a method for measuring the NOx concentration in which the O2 concentration is adjusted by using a first pumping cell arranged in the first internal space, and NO is decomposed by using a decomposing pump arranged in the second internal space so that the NOx concentration is measured on the basis of a pumping current flowing through the decomposing pump.
Further, Japanese Laid-Open Patent Publication No. 9-113484 discloses a sensor element comprising an auxiliary pumping electrode arranged in a second internal space so that the oxygen concentration in the second internal space is controlled to be constant even when the oxygen concentration is suddenly changed.
The following fact has been revealed when a gas sensor is attached to an exhaust system of an internal combustion engine such as an automobile engine, and the internal combustion engine is operated. That is, in ordinary cases, the sensor output ordinarily makes proportional change based on an anchoring point of zero in accordance with the change in oxygen concentration as shown by a solid line xe2x80x9caxe2x80x9d in FIG. 32. However, under a specified operation condition, the sensor output is subjected to shift-up as a whole as shown by a solid line xe2x80x9cbxe2x80x9d.
In general, as shown in FIG. 33, the total pressure of the exhaust gas discharged from the automobile engine is composed of a constant static pressure and a dynamic pressure generated by the pulsation of the exhaust gas pressure. The fluctuation cycle of the dynamic pressure is synchronized with the explosion cycle of the engine. As a result of investigation on the cause of the shift-up of the sensor output, it has been revealed that the shift-up occurs when the pulsation amount (=dynamic pressure) of the exhaust gas pressure is large as compared with the static pressure.
That is, as shown in FIG. 34, the shift amount of the sensor output has been measured with respect to the ratio between the dynamic pressure and the static pressure (dynamic pressure/static pressure). As a result, the shift amount is approximately zero when the dynamic pressure/static pressure is not more than about 25%. However, the shift amount increases proportionally from the stage at which the dynamic pressure/static pressure exceeds about 25%.
Therefore, when the dynamic pressure is increased, it is inevitable to suffer any deterioration of the correlation between the oxygen-pumping amount effected by the main pump in the first space and the oxygen concentration in the measurement gas. It is feared that the disturbance of the oxygen concentration caused in the first space may bring about any deterioration concerning the control of the oxygen concentration in the second space communicating with the first space and the accuracy of measurement effected by the detecting electrode as the NOx-detecting section.
The present invention has been made taking such problems into consideration, an object of which is to provide a gas sensor and a nitrogen oxide sensor which make it possible to avoid the influence of the pulsation of the exhaust gas pressure generated in the measurement gas, and improve the measurement accuracy obtained on the detecting electrode.
According to the present invention, there is provided a gas sensor for measuring an amount of a measurement gas component contained in a measurement gas existing in external space; the gas sensor at least comprising a substrate composed of a solid electrolyte to make contact with the external space; an internal space formed at the inside of the substrate; a diffusion rate-determining means formed with a slit for introducing the measurement gas from the external space via a gas-introducing port under a predetermined diffusion resistance; and a pumping means including an inner pumping electrode and an outer pumping electrode formed at the inside and outside of the internal space respectively, for pumping-processing oxygen contained in the measurement gas introduced from the external space, on the basis of a control voltage applied between the electrodes; wherein a dimension of a certain factor for forming a cross-sectional configuration of the diffusion rate-determining means is not more than 10 xcexcm.
The limiting current value Ip in the pumping means is approximated by the following theoretical expression for the limiting current.
Ip≈(4F/RT)xc3x97Dxc3x97(S/L)xc3x97(POexe2x88x92POd) 
In the expression, F represents the Faraday constant (=96500 A/sec), R represents the gas constant (=82.05 cm3xe2x80xa2atm/molxe2x80xa2K), T represents the absolute temperature (K), D represents the diffusion coefficient (cm2/sec), S represents the cross-sectional area (cm2) of the diffusion rate-determining means, L represents the passage length (cm) of the diffusion rate-determining means, POe represents the partial pressure of oxygen (atm) at the outside of the diffusion rate-determining means, and POd represents the partial pressure of oxygen (atm) at the inside of the diffusion rate-determining means.
The present invention defines the factor for forming the cross-sectional area S of the diffusion rate-determining means in the theoretical limiting current expression. Especially, it is defined that the certain factor of the dimension for forming the cross-sectional area S is not more than 10 xcexcm.
In this arrangement, the pulsation (=dynamic pressure) of the exhaust gas pressure is attenuated by the wall resistance of the diffusion rate-determining means. Specifically, the attenuation is effected up to the level at which the ratio between the dynamic pressure and the static pressure (dynamic pressure/static pressure) is not more than 25%. Therefore, it is possible to effectively suppress the shift-up phenomenon of the sensor output which would be otherwise caused by the fluctuation of the dynamic pressure.
It is also preferable for the gas sensor constructed as described above that when the cross-sectional configuration of the diffusion rate-determining means is formed with at least one lateral type slit, the certain factor is a length of the slit in a vertical direction. Alternatively, it is also preferable that when the cross-sectional configuration of the diffusion rate-determining means is formed with at least one vertical type slit, the certain factor is a length of the slit in a lateral direction.
It is also preferable for the gas sensor constructed as described above that a buffering space is provided between the gas-introducing port and the diffusion rate-determining means. Usually, the oxygen suddenly enters the sensor element via the gas-introducing port due to the pulsation of the exhaust gas pressure brought about in the external space. However, in this arrangement, the oxygen from the external space does not enter the processing space directly, but it enters the buffering space disposed at the upstream stage thereof. In other words, the sudden change in oxygen concentration, which is caused by the pulsation of the exhaust gas pressure, is counteracted by the buffering space. Thus, the influence of the pulsation of the exhaust gas pressure, which is exerted on the internal space, is in an almost negligible degree.
As a result, the correlation is improved between the oxygen-pumping amount effected by the pumping means in the processing space and the oxygen concentration in the measurement gas. It is possible to improve the measurement accuracy obtained on the measuring pumping means or the concentration-detecting means. Simultaneously, for example, it is possible to concurrently use the internal space as a sensor for determining the air-fuel ratio.
It is also preferable for the gas sensor concentrated as described above that a clogging-preventive section and the buffering space are provided in series between the gas-introducing port and the internal space (processing space); a front aperture of the clogging-preventive section is used to form the gas-introducing port; and a diffusion rate-determining section for giving a predetermined diffusion resistance to the measurement gas is provided between the clogging-preventive section and the buffering space.
In this arrangement, the gas sensor is prevented from clogging of particles (for example, soot and oil combustion waste) produced in the measurement gas in the external space, which would be otherwise caused in the vicinity of the inlet of the buffering space. Thus, it is possible to measure the predetermined gas component more accurately. Further, it is possible to maintain a highly accurate state over a long period of time.
It is also preferable for the gas sensor constructed as described above that the oxygen contained in the measurement gas introduced from the external space into the internal space is pumping-processed by using the pumping means to make control so that a partial pressure of oxygen in the internal space (processing space) has a predetermined value at which a predetermined gas component in the measurement gas is not decomposable.
It is also preferable that the gas sensor further comprises a measuring pumping means for decomposing the predetermined gas component contained in the measurement gas after being pumping-processed by the pumping means, by means of catalytic action and/or electrolysis, and pumping processing oxygen produced by the decomposition; wherein the predetermined gas component contained in the measurement gas is measured on the basis of a pumping current flowing through the measuring pumping means in accordance with the pumping process effected by the measuring pumping means.
Alternatively, it is also preferable that the gas sensor further comprises an oxygen partial pressure-detecting means for decomposing the predetermined gas component contained in the measurement gas after being pumping-processed by the pumping means, by means of catalytic action, and generating an electromotive force corresponding to a difference between an amount of oxygen produced by the decomposition and an amount of oxygen contained in a reference gas; wherein the predetermined gas component contained in the measurement gas is measured on the basis of the electromotive force detected by the oxygen partial pressure-detecting means.
According to another aspect of the present invention, there is provided a nitrogen oxide sensor for measuring an amount of a nitrogen oxide component contained in a measurement gas existing in external space; the nitrogen oxide sensor at least comprising a substrate composed of an oxygen ion-conductive solid electrolyte to make contact with the external space; a first internal space formed at the inside of the substrate and communicating with the external space; a first diffusion rate-determining means formed with a slit for introducing the measurement gas into the first internal space under a predetermined diffusion resistance; a main pumping means including a first inner pumping electrode and a first outer pumping electrode formed at the inside and outside of the first internal space respectively, for pumping-processing oxygen contained in the measurement gas introduced from the external space, on the basis of a control voltage applied between the electrodes so that a partial pressure of oxygen in the first internal space is controlled to have a predetermined value at which NO is not substantially decomposable; a second internal space communicating with the first internal space; a second diffusion rate-determining means formed with a slit for introducing an atmosphere pumping-processed in the first internal space into the second internal space under a predetermined diffusion resistance; and a measuring pumping means including a second inner pumping electrode and a second outer pumping electrode formed at the inside and outside of the second internal space respectively, for decomposing NO contained in the atmosphere introduced from the first internal space, by means of catalytic action and/or electrolysis to pumping-process oxygen produced by the decomposition; wherein the amount of nitrogen oxide contained in the measurement gas is measured on the basis of a pumping current flowing through the measuring pumping means in accordance with the pumping process effected by the measuring pumping means; and a dimension of a certain factor for forming a cross-sectional configuration of at least one of the diffusion rate-determining means is not more than 10 xcexcm.
According to the present invention, the pulsation (=dynamic pressure) of the exhaust gas pressure is attenuated by the wall resistance of the diffusion rate-determining means. Specifically, the attenuation is effected up to the level at which the ratio between the dynamic pressure and the static pressure (dynamic pressure/static pressure) is not more than 25%. Therefore, it is possible to effectively suppress the shift-up phenomenon of the sensor output which would be otherwise caused by the fluctuation of the dynamic pressure.
According to still another aspect of the present invention, there is provided a nitrogen oxide sensor for measuring an amount of a nitrogen oxide component contained in a measurement gas existing in external space; the nitrogen oxide sensor at least comprising a substrate composed of an oxygen ion-conductive solid electrolyte to make contact with the external space; a first internal space formed at the inside of the substrate and communicating with the external space; a first diffusion rate-determining means formed with a slit for introducing the measurement gas into the first internal space under a predetermined diffusion resistance; a main pumping means including an inner pumping electrode and an outer pumping electrode formed at the inside and outside of the first internal space respectively, for pumping-processing oxygen contained in the measurement gas introduced from the external space, on the basis of a control voltage applied between the electrodes so that a partial pressure of oxygen in the first internal space is controlled to have a predetermined value at which NO is not substantially decomposable; a second internal space communicating with the first internal space; a second diffusion rate-determining means formed with a slit for introducing atmosphere pumping-processed in the first internal space into the second internal space under a predetermined diffusion resistance; and an oxygen partial pressure-detecting means including an inner measuring electrode and an outer measuring electrode formed at the inside and outside of the second internal space respectively, for decomposing NO contained in the atmosphere introduced from the first internal space, by means of catalytic action to generate an electromotive force corresponding to a difference between an amount of oxygen produced by the decomposition and an amount of oxygen contained in a reference gas; wherein the amount of nitrogen oxide contained in the measurement gas is measured on the basis of the electromotive force detected by the oxygen partial pressure-detecting means; and a dimension of a certain factor for forming a cross-sectional configuration of at least one of the diffusion rate-determining means is not more than 10 xcexcm.
Also in this aspect, the pulsation (=dynamic pressure) of the exhaust gas pressure is attenuated by the wall resistance of the diffusion rate-determining means. Specifically, the attenuation is effected up to the level at which the ratio between the dynamic pressure and the static pressure (dynamic pressure/static pressure) is not more than 25%. Therefore, it is possible to effectively suppress the shift-up phenomenon of the sensor output which would be otherwise caused by the fluctuation of the dynamic pressure.